Tethlis loathed the Druchii with a passion, for they had murdered his family when he was but a child. At the start of his reign, the Dragons of Caledor began to succumb to a strange malaise, making them difficult to summon to war or even awaken from deep slumbers, and so Tethlis had to find new ways to compensate for the loss of these mighty beasts in his army. He began to train more and more soldiers, until he had rebuilt the Asur armies to a strength not seen since the time of Aenarion.

When this was complete, Tethlis launched the Scouring, a long campaign to drive the Dark Elves out of Ulthuan. The armies of Ulthuan and Naggaroth waged war against each other for centuries, the course of the war changing on the outcome of a single battle. The war reached heights of ferocity not seen since the days of the Sundering, as seen at the siege of Griffon Gate, where a Druchii army was ambushed by Tethlis' forces: only a counter-attack led by Malekith himself was enough to save the Dark Elves from destruction, or the siege of Tor Lehan, where the Asur and Druchii forces wiped each other out down to the last Elf, leaving no survivors. After many years, Tethlis pushed the Druchii back into Nagarythe. He laid siege to Anlec and when it fell, gave the order for all Druchii prisoners -men, women and children- to be put to death - an action which only inflamed the hatred of the Dark Elves. Tethlis' commanders were also worried that the Phoenix King's hatred and bloodlust would spread to the Elven armies until they became no better than what they fought against.

On the 303rd year of his rule, he led a great armada to the Blighted Isle where he inflicted a mighty defeat upon the Druchii: he also found the armour of Aenarion himself and had the great heirloom passed onto the descendants of Morelion, Aenarion's son by his first wife. He later died under mysterious circumstances: some claimed that he was slain by a Dark Elf Assassin hiding among the bones surrounding the Sword of Khaine, others that he attempted to draw the weapon and as it began to come free, he was murdered by his own bodyguard, who feared the unleashing of the sword's terrible power. Whatever the truth of the matter, Tethlis did not return from the Blighted Isle and the Asur armada sailed home.